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Register a new userEvidence of Spontaneous Vortex Ground State in An Iron-Based Ferromagnetic Superconductor
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Spontaneous vortex phase (SVP) is an exotic quantum matter in which quantized superconducting vortices form in the absence of external magnetic field. Although being predicted theoretically nearly 40 years ago, its rigorous experimental verification still appears to be lacking. Here we present low-field magnetic measurements on single crystals of the iron-based ferromagnetic superconductor Eu(Fe$_{0.91}$Rh$_{0.09}$)$_{2}$As$_{2}$ which undergoes a superconducting transition at $T_mathrm{sc}$ = 19.6 K followed by a magnetic transition at $T_mathrm{m}$ = 16.8 K. We observe a characteristic first-order transition from a Meissner state within $T_mathrm{m}<T<T_mathrm{sc}$ to an SVP below $T_mathrm{m}$, under a magnetic field approaching zero. Additional isothermal magnetization and ac magnetization measurements at $Tll T_mathrm{sc}$ confirm that the system is intrinsically in a spontaneous-vortex ground state. The unambiguous demonstration of SVP in the title material lays a solid foundation for future imaging and spectroscopic studies on this intriguing quantum matter.
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The search for Majorana bound state (MBS) has recently emerged as one of the most active research areas in condensed matter physics, fueled by the prospect of using its non-Abelian statistics for robust quantum computation. A highly sought-after platform for MBS is two-dimensional topological superconductors, where MBS is predicted to exist as a zero-energy mode in the core of a vortex. A clear observation of MBS, however, is often hindered by the presence of additional low-lying bound states inside the vortex core. By using scanning tunneling microscope on the newly discovered superconducting Dirac surface state of iron-based superconductor FeTe1-xSex (x = 0.45, superconducting transition temperature Tc = 14.5 K), we clearly observe a sharp and non-split zero-bias peak inside a vortex core. Systematic studies of its evolution under different magnetic fields, temperatures, and tunneling barriers strongly suggest that this is the case of tunneling to a nearly pure MBS, separated from non-topological bound states which is moved away from the zero energy due to the high ratio between the superconducting gap and the Fermi energy in this material. This observation offers a new, robust platform for realizing and manipulating MBSs at a relatively high temperature.
The mechanism of the interplay between superconductivity and magnetism is one of the intriguing and challenging problems in physics. Theory has predicted that the ferromagnetic order can coexist with the superconducting order in the form of a spontaneous vortex phase in which magnetic vortices nucleate in the absence of an external field. However, there has been no rigorous demonstration of spontaneous vortices by bulk magnetic measurements. Here we show the results of experimental observations of spontaneous vortices using a superconductor/ferromagnet fractal nanocomposite, in which superconducting MgB2 and ferromagnetic nanograins are dispersedly embedded in the normal matrix to realize the remote electromagnetic interaction and also to induce a long-range Josephson coupling. We found from bulk magnetization measurements that the sample with nonzero remanent magnetization exhibits the magnetic behaviors which are fully consistent with a spontaneous vortex scenario predicted theoretically for magnetic inclusions in a superconducting material. The resulting spontaneous vortex state is in equilibrium and coexists surprisingly with a Meissner state (complete shielding of an external magnetic field). The present observation not only reveals the evolution process of the spontaneous vortices in superconductor/ferromagnet hybrids, but it also sheds light on the role of the fractal disorder and structural heterogeneity on the vortex nucleation under the influence of Josephson superconducting currents.
Majorana zero-modes (MZMs) are spatially-localized zero-energy fractional quasiparticles with non-Abelian braiding statistics that hold a great promise for topological quantum computing. Due to its particle-antiparticle equivalence, an MZM exhibits robust resonant Andreev reflection and 2e2/h quantized conductance at low temperature. By utilizing variable-tunnel-coupled scanning tunneling spectroscopy, we study tunneling conductance of vortex bound states on FeTe0.55Se0.45 superconductors. We report observations of conductance plateaus as a function of tunnel coupling for zero-energy vortex bound states with values close to or even reaching the 2e2/h quantum conductance. In contrast, no such plateau behaviors were observed on either finite energy Caroli-de Genne-Matricon bound states or in the continuum of electronic states outside the superconducting gap. This unique behavior of the zero-mode conductance reaching a plateau strongly supports the existence of MZMs in this iron-based superconductor, which serves as a promising single-material platform for Majorana braiding at a relatively high temperature.
Using low-temperature Magnetic Force Microscopy (MFM) we provide direct experimental evidence for spontaneous vortex phase (SVP) formation in EuFe$_2$(As$_{0.79}$P$_{0.21}$)$_2$ single crystal with the superconducting $T^{rm 0}_{rm SC}=23.6$~K and ferromagnetic $T_{rm FM}sim17.7$~K transition temperatures. Spontaneous vortex-antivortex (V-AV) pairs are imaged in the vicinity of $T_{rm FM}$. Also, upon cooling cycle near $T_{rm FM}$ we observe the first-order transition from the short period domain structure, which appears in the Meissner state, into the long period domain structure with spontaneous vortices. It is the first experimental observation of this scenario in the ferromagnetic superconductors. Low-temperature phase is characterized by much larger domains in V-AV state and peculiar branched striped structures at the surface, which are typical for uniaxial ferromagnets with perpendicular magnetic anisotropy (PMA). The domain wall parameters at various temperatures are estimated.
Neutron diffraction and small angle scattering experiments have been carried out on the double-isotopic polycrystalline sample (7Li0.82Fe0.18OD)FeSe. Profile refinements of the diffraction data establish the composition and reveal an essentially single phase material with lattice parameters of a= 3.7827 {AA} and c= 9.1277 {AA} at 4 K, in the ferromagnetic-superconductor regime, with a bulk superconducting transition of TC = 18 K. Small angle neutron scattering (SANS) measurements in zero applied field reveal the onset of ferromagnetic order below TF ~ 12.5 K, with a wave vector and temperature dependence consistent with an inhomogeneous ferromagnet of spontaneous vortices or domains in a mixed state. No oscillatory long range ordered magnetic state is observed. Field dependent measurements establish a separate component of magnetic scattering from the vortex lattice, which occurs at the expected wave vector. The temperature dependence of the vortex scattering does not indicate any contribution from the ferromagnetism, consistent with diffraction data that indicate that the ordered ferromagnetic moment is quite small.
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